1. Field of the Invention
The present invention relates generally to Raman amplification, and particularly to distributed Raman amplifiers.
2. Technical Background
A typical optical communication system utilizes a plurality of discrete amplifiers situated between lengths of transmission fiber. The discrete amplifiers are usually Erbium doped fiber amplifiers. The distance between the Erbium doped fiber amplifiers is often referred to as the xe2x80x9cspan lengthxe2x80x9d. It has been known that an optical communication system can utilize distributed Raman fiber amplifiers in conjunction with the Erbium doped fiber amplifiers. Distributed Raman fiber amplifiers utilize the transmission fiber as their gain medium. Distributed Raman amplification has been shown to improve the performance of optical communication systems by reducing the effective noise figure (NF) of the inline amplifiers.
When distributed Raman fiber amplifiers are utilized in conjunction with the Erbium doped fiber amplifiers, the number of spans in an optical communication system can be increased. In addition, the span length between the Erbium doped amplifiers can be significantly increased. This enables systems with larger amplifier spacing, longer reach, higher bit rates, or wider bandwidth. These important advantages come from the fact that the communication fiber itself can provide not only signal attenuation, but also, in the presence of a strong pump, signal amplification.
For small Raman gains, the optical signal-to-noise ratio (OSNR) and the effective NF improve with increasing the Raman gain. At higher gains, however, this trend is reversed by the effects of Rayleigh scattering of amplified spontaneous emission (RSASE) and by multipath interference due to the double Rayleigh back scattering of the signal (MPI due to DRBS). As high power pumps, which enable higher Raman gain, become increasingly commercially available, these performance-related limitations on the amount of useful Raman gain become increasingly important.
One aspect of the invention is a distributed Raman amplification system in which the pump laser or lasers and the fiber have characteristics that result in broadening of the DRBS spectrum. In another aspect, the invention is a Raman amplification system with a transmission fiber through which signal light of a wavelength xcexs propagates, and having zero dispersion at a wavelength xcexo; a pump laser coupled to said fiber and producing pump light at a wavelength xcexp; where xcexp and xcexs are on opposite sides of the zero dispersion wavelength (i.e., xcexp less than xcexo less than xcexs).
In another aspect, the invention is such a system wherein the transmission fiber is large effective area fiber.
In another aspect, the invention is a Raman amplification system in which the pump spectral bandwidth is greater than the signal spectral bandwidth.
In another aspect, the invention is a Raman amplification system suitable for use in a WDM optical fiber communication system, which includes a transmission fiber through which signal light propagates, and a pump laser coupled to the fiber, where the pump mode spacing is less than the optical bandwidth of the signal, or where the relationship between pump mode spacing xcex94vpump, WDM channel spacing xcex94vsig and optical bandwidth of the signal BO is such that the inequality |mxcex94vpumpxe2x88x92nxcex94vsig| greater than BO ceases to be true for |m| and |n| larger than xcex94vsig/BO.
In these various embodiments, the invention is advantageous in that it lessens the amount of system degradation due to DRBS. In certain practical applications, it is possible to lessen DRBS from consideration, and it may be possible to eliminate or nearly eliminate it in some instances.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely illustrative of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.